Zemanek, U.S. Pat. No. 3,369,626 discloses an ultrasonic tool for use in scanning the inner surface of an open well borehole or of casing in a borehole. The tool, which is commercially known as the "borehole televiewer", creates a high resolution picture of the inner surface under investigation. The borehole televiewer is used to "see" the inner surface under investigation through drilling mud or other borehole fluids. In an open borehole, the borehole televiewer provides a picture of the formations surrounding the borehole. In a cased borehole, the borehole televiewer provides a picture of the inner surface of the casing, which can be used to determine the condition of the inner surface.
The borehole televiewer uses a rotating ultrasonic transducer. The transducer serves as a transmitter, to generate acoustic waveforms, and a receiver, to receive the acoustic return. The acoustic return is caused by the reflection of the generated acoustic waveform from the inner surface under investigation. The acoustic return has two measured parameters, the time of travel of the acoustic return and the amplitude, which give an indication of the condition of the investigated surface.
The transducer rotates about three revolutions per second, is pulsed about 500 times per revolution, and is pulled up the borehole at a speed of about 5 feet per second. The ultrasonic transducer spot size, the rotational speed, the pulse repetition rate, and the vertical speed combine to provide full coverage of the investigated inner surface, resulting in high areal resolution of the inner surface.
The high resolution logging operations generate a tremendous amount of data. The data is transmitted uphole over a logging cable. Unfortunately, the logging cable has a limited bandwidth, thereby creating a potential bottleneck.
The borehole televiewer solves the problem of transmitting large amounts of data over the logging cable by transmitting the envelope of the acoustic return. The amplitude of the acoustic return is contained within the envelope information. The time of travel of the acoustic return is encoded by transmitting the envelope over the logging cable at a time interval from a synchronization pulse. The time interval corresponds to the time of travel of the acoustic return. The synchronization pulse is synchronous to the generated acoustic waveform from the ultrasonic transducer.
The surface electronics measures the amplitude of the acoustic envelope and the time interval of the acoustic envelope from the synchronization pulse. However, the logging cable degrades the rise time of the acoustic envelope , thereby affecting the time interval measurement by the surface electronics.
What is needed is a method of transmitting time of travel and amplitude information from an ultrasonic tool to surface electronics over a logging cable, which information is not degraded by the logging cable.